Continuous process for controlled evaporation of black liquor

ABSTRACT

Described is a process for concentrating weak black liquor to form strong black liquor, said process comprising: a) preconcentrating said weak black liquor; b) heating said preconcentrated weak black liquor in a heating zone under sufficient pressure to prevent said preconcentrated weak black liquor from boiling in said heating zone; c) passing said preconcentrated weak black liquor, having at least one volatile component, into an evaporation zone through one or more orifices; d) applying pressure in said evaporation zone that is lower than the vapor pressure of said preconcentrated weak black liquor as it is passed into said evaporation zone, which allows for flash evaporation of at least a portion of said at least one volatile component of said preconcentrated weak black liquor from said preconcentrated weak black liquor to form strong black liquor; e) adjusting heat in said heating zone., pressure in said heating zone and pressure in said evaporation zone to allow for evaporation of an amount of said at least one volatile component from said preconcentrated weak black liquor to form said strong black liquor; and f) collecting said strong black liquor from said evaporation zone.

FIELD OF THE INVENTION

[0001] The present invention relates generally to continuous pulpproduction. More particularly, the invention relates to a method forevaporating weak black liquor to form strong black liquor.

BACKGROUND OF THE INVENTION

[0002] In the pulping process, it is found that by separating spentpulping liquor (known as “weak black liquor”) from the fibrous plantmaterial, the purity of the pulp is improved. Weak black liquortypically contains 15 wt % dissolved and suspended solids of which about80% are organic compounds and the remainder are inorganic compounds. Theweak black liquor is separated from the pulp at a certain point in theprocess of pulp production. The weak black liquor is then subjected toevaporation to remove water. The weak black liquor, after it isconcentrated, is then called “strong black liquor.” The strong blackliquor is then passed to a recovery boiler where the strong black liquoris combusted to recover inorganics, such as sulfur and sodium, for reusein the pulping process. The steam that is produced by the combustion ofthe strong black liquor is used for heating in the pulping process.

[0003] The solids content in the strong black liquor must lie in adesired range to ensure proper burning in the recovery boiler. If toomuch water is evaporated, viscous strong black liquor may plug the spraynozzles or simply spray improperly, possibly extinguishing the burnerflame. Conversely, if too little water is evaporated from the weak blackliquor, the organics content will be too low to sustain a flame. Eithercase is dangerous, because the unburned strong black liquor would thenfall into a pool of molten inorganic chemicals, which lies at the bottomof the recovery boiler, possibly causing an explosion. System interlocksare relied upon to shut off the flow of strong black liquor to theburner nozzles if the flame goes out.

[0004] Processes for the evaporation of weak black liquor to form strongblack liquor are known in the art, such as direct contact evaporators,indirectly-heated concentrators, and multi-effect evaporators. However,the need still exists for a process that can control the concentrationof strong black liquor reliably, consistently and precisely.

SUMMARY OF THE INVENTION

[0005] The present inventive process provides a controlled method forevaporating weak black liquor to form strong black liquor.Advantageously, the method offers fast and precise control of viscosityof the strong black liquor. The control scheme allows for regulation ofthe concentration of the outgoing product to tight tolerances bymanipulating the system pressure and the temperature of the incomingblack liquor. It allows for good control over the product percentsolids, and the product viscosity. Precisely controlling the viscosityand solids content of the strong black liquor sent to the recoveryboiler would not only improve safety, but would also allow optimizationof the operation of the recovery boiler, with respect to both chemicalrecovery capacity and energy efficiency.

[0006] The inventive process is a process for concentrating weak blackliquor to form strong black liquor, said process comprising: a)preconcentrating said weak black liquor; b) heating said preconcentratedweak black liquor in a heating zone under sufficient pressure to preventsaid preconcentrated weak black liquor from boiling in said heatingzone; c) passing said preconcentrated weak black liquor having a vaporpressure and having at least one volatile component, into an evaporationzone through one or more orifices; d) applying pressure in saidevaporation zone that is lower than the vapor pressure of saidpreconcentrated weak black liquor as it is passed into said evaporationzone, which allows for flash evaporation of at least a portion of saidat least one volatile component of said preconcentrated weak blackliquor from said preconcentrated weak black liquor to form strong blackliquor; e) adjusting temperature in said heating zone, pressure in saidheating zone and pressure in said evaporation zone to allow forevaporation of an amount of said at least one volatile component fromsaid preconcentrated weak black liquor to form said strong black liquor;and f) collecting said strong black liquor from said evaporation zone.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007]FIG. 1 is a process flow diagram of an embodiment of the presentinventive method; and

[0008]FIG. 2 is a graph showing evaporator response time for sugarsolution, with the normalized change in % solids versus the time, inminutes, for such a change in pressure (two changes) and temperature(two changes).

DETAILED DESCRIPTION OF THE INVENTION

[0009] An embodiment of the present invention is illustrated in FIG. 1by a process flow diagram. In this embodiment, weak black liquor fromthe wood pulping and washing steps of a pulp production process ispreconcentrated in a preconcentrator 12. The bulk of the water, aqueousmedia or solvent media in the weak black liquor is removed in this step.

[0010] The preconcentrated weak black liquor is then pumped, using apump 14, to a heating zone 16 where it is heated. A temperature sensor18 monitors or senses the temperature of the heating zone 16. Thepreconcentrated weak black liquor is kept under sufficient temperatureand pressure in the heating zone 16 to keep the preconcentrated weakblack liquor from boiling there.

[0011] There is a pressure sensor 20 and another temperature sensor 22in the system prior to an evaporation zone (a “zone” is a chamber,enclosed area, etc.) 24, which are used to sense the pressure and thetemperature of the preconcentrated weak black liquor just prior to thepreconcentrated weak black liquor entering the evaporation zone 24. Thepreconcentrated weak black liquor is then passed into the evaporationzone 24 through an inlet 26 and a die 28 comprising one or moreorifices. The die 28 is constructed such that the pressure drop throughthe die should prevent the preconcentrated weak black liquor fromboiling until it has nearly entered, or has actually entered theevaporation zone. In order for the flash evaporation of thepreconcentrated weak black liquor to take place in the evaporation zone24, pressure is controlled in the evaporation zone 24 such that it islower than the vapor pressure of the heated preconcentrated weak blackliquor as it enters the evaporation zone 24. The strong black liquor iscooled as it passes through the evaporation zone 24 and then goesthrough an outlet 30.

[0012] A vapor exit port 34 is attached to the evaporation zone 24 wherethe vapor that was evaporated out of the strong black liquor in theevaporation zone 24 is removed from the evaporation zone 24. Anotherpressure sensor 36 is located outside of the vapor exit port 34 or couldbe somewhere connected to the evaporation zone 24 itself, and is used tosense the pressure in the evaporation chamber 24 or just outside of it.In order for the vapor to exit the evaporation zone 24 the pressureneeds to be lower outside of the evaporation zone 24 than inside. Thelower pressure outside of the evaporation zone 24 may be provided by avacuum system 40 that is connected to the system via a condenser 38where most of the vapor is condensed for removal. Alternatively, vaporsmay pass directly to the pressure control or the vacuum system.

[0013] The strong black liquor is collected in a collection vessel 42.It is then pumped out of the system by a pump 44. A conditions sensor 46senses, measure or infers such things as viscosity, pressure,concentration and heating value of the strong black liquor and is foundat or near the exit for the strong black liquor or is attached to thecollection vessel. These measurements or inferences can be used incontrolling the operation of the system.

[0014] Preconcentrating

[0015] The preconcentrating step may be performed using apreconcentrator 12. Apparatuses that may be used as preconcentratorsinclude, for example, multi-effect evaporators.

[0016] “Preconcentrating” means removing a portion of at least onevolatile component in a solution or liquid medium prior to the firststep of a process.

[0017] The volatile component of the weak black liquor that is removedor evaporated is mostly or all water. Other ingredients in the weakblack liquor may also be evaporated, although that is not mostdesirable.

[0018] Heating

[0019] The heating step of the present inventive method is performed ina heating zone 16. The heating is preferably done using a heatexchanger. The heating step may be performed, however, by any suitableheating means or device. Some examples of other heating means include,but are not limited to, direct steam injection and microwaves.

[0020] The preconcentrated weak black liquor is heated to a temperaturein the heating zone that is suitable so that the preconcentrated weakblack liquor will have a temperature that is above the boiling point ofthe preconcentrated weak black liquor at the pressure in the evaporationzone.

[0021] The pressure in the heating zone is maintained so that thepreconcentrated weak black liquor is not allowed to boil. The range ofpressures depends upon the composition of the preconcentrated weak blackliquor and the temperature of the heating zone and of thepreconcentrated weak black liquor.

[0022] Appropriate temperature and pressure may be calculated usingstandard thermodynamic relationships. Methods for doing thesecalculations can be found in references such as R. C. Reid et al., TheProperties of Gases and Liquids, 4^(th) Ed., McGraw-Hill, New York,N.Y., 1987, Chapters 7 and 8, and Appendix A.

[0023] Evaporation

[0024] After being heated in the heating zone 16, the preconcentratedweak black liquor is passed through an inlet or conduit and through adie 28 or nozzle or some sort of orifice into the evaporation zoneitself. The die or nozzle may be of any suitable shape and configurationthat allow the heated preconcentrated weak black liquor to pass throughinto the evaporation zone 24. The shape and/or number of orifice(s)through which it passes may allow for increase in the surface area ofthe preconcentrated weak black liquor that is exposed in the evaporationzone as it is passed through the zone. The preconcentrated weak blackliquor is passed through into the evaporation zone 24 and is flashevaporated. Preferably, the die 28, which is a device having a specificshape or design that it imparts to such material that is passed throughit, is of a suitable shape or configuration that allows thepreconcentrated weak black liquor to pass through the evaporation zone24 and be flash evaporated to form strong black liquor. Some die designswill have the added property that the preconcentrated weak black liquorwill pass through the evaporation zone 24 and be flash evaporated toform strong black liquor without substantially contacting the innersurface or surfaces 32 of the evaporation zone 24. In some operationsthis is desirable.

[0025] An exemplary die has a plurality of orifices or holes that allowthe preconcentrated weak black liquor to form strands or streams (thatmay be discontinuous) as the preconcentrated weak black liquor passesthrough the evaporation zone 24. Another possibility is for the die 28to have orifices that are slits and allow for sheets or curtains (thatmay be discontinuous) of heated preconcentrated weak black liquor totravel or fall into the evaporation chamber 24. These exemplary designsof the die 28 allow more surface area of preconcentrated weak blackliquor to be exposed in the evaporation zone 24, which allows for moreefficient evaporation. Other configurations of the die and its orificeor orifices are also contemplated for this invention.

[0026] The at least one volatile component (volatile component beingvolatile at the temperature at which the preconcentrated weak blackliquor enters the evaporation zone and at the pressure in theevaporation zone) that is evaporated from the preconcentrated weak blackliquor to form the strong black liquor is mostly, if not all, water.Other ingredients of the preconcentrated weak black liquor may beevaporated, although that is not most desirable.

[0027] The evaporation step relies on the mechanism wherein the energyabsorbed by the preconcentrated weak black liquor during the heatingstep is preserved by maintaining a back pressure on the preconcentratedweak black liquor to prevent vaporization of the preconcentrated weakblack liquor in the inlet 26 to the evaporation zone 24, and issubsequently released in the evaporation zone causing evaporation. Thenumber and size of the orifices in the die 28 affect the back pressurethat is maintained upstream. The back pressure must be sufficient tokeep the preconcentrated weak black liquor from boiling before it entersthe evaporation zone 24.

[0028] A multi-orifice die may be preferred, particularly because theweak black liquor is to be concentrated or evaporated in a single passthrough the evaporation zone. Selection of the number and size of theorifices is used to achieve the desired pressure to prevent boiling ofthe preconcentrated weak black liquor prior to entering the evaporationzone.

[0029] The pressure in the evaporation zone 24 is controlled so thatwhen the preconcentrated weak black liquor enters the evaporation zone24 it flash evaporates. The greater the temperature differential of thepreconcentrated weak black liquor, the faster will be the rate ofevaporation. Flash evaporation is an elementary step in which volatilecomponents, for example, unreacted monomers, solvents or species, areremoved. In the case of weak black liquor, the change in pressure allowsfor flash evaporation of the volatile component(s) from the weak blackliquor, resulting in concentration to form strong black liquor.

[0030] The desired level of pressure necessary in the process may becalculated using, for example, the calculations set forth in Reid etal., The Properties of Gases and Liquids, McGraw-Hill Book Co., 4^(th)ed., Chapters 7 and 8, and Appendix A.

[0031] There may be many factors that interact in selecting the targetstrong black liquor concentration and the present invention can beemployed to achieve whatever increased concentration (or level ofevaporation) is finally selected relative to the initial concentration.

[0032] The strong black liquor will generally have a percent solids ofabout 45-50 wt % solids before it is fed to the recovery boiler.

[0033] Continuous Adjustment

[0034] The pressure and/or temperature of the inventive process areperiodically or continuously controlled and manually or automaticallycontrolled. This is done using a plurality of temperature and pressuresensors, as described above. In addition, a conditions sensor may beused to monitor the conditions or properties of the remaining (orpost-evaporation) strong black liquor. The sensors may be part of acontrol system(s) that can regulate the pressure and temperature of thepre-evaporation black liquor, evaporation zone, etc. In an exemplaryembodiment, one or more of the variables in the inventive process(pressures, temperatures) are regulated by automatic control systems,the set points having been manually supplied by an operator. In anotherexemplary embodiment, one or more of the closed-loop systems thatregulate temperature and pressure will receive set points from anothercontrol system, based on directly measured or inferred properties of thepost-evaporation strong black liquor. In another exemplary embodiment,the entire inventive process will be controlled by an automatic controlsystem, based on measurements of temperature and/or pressure, and onproperties of the post-evaporation strong black liquor. The operatorwill specify a desired property (such as percent solids content) of thepost-evaporation strong black liquor, and the control system willautomatically regulate process variables in such a way as to achieve thedesired property.

[0035] The control system may comprise a single multivariablecontroller, or several independent single-loop controllers, or acombination of controller types.

[0036] Collection

[0037] The concentrated black liquor is collected in a collection vessel42. A pump may be mounted at, in, or after the collection vessel. A pumpshould be selected to be of a type that can remove strong black liquorfrom the collection vessel and pass it at a higher pressure into anultimate collection vessel of some type, or it could be passed to therecovery boiler (in pulp production process) from the collection vessel.This may be done continuously or periodically.

EXAMPLES

[0038] This invention is further illustrated by the following examples,which are not intended to limit the scope of the invention. In theexamples, sucrose solutions were used in place of black liquor. Blackliquor is difficult to obtain for laboratory studies unless one isclosely associated with a pulp manufacturing plant. It is expected thatthe sucrose solutions mimicked the behavior of black liquor in theinventive process. In the examples, all parts, ratios and percentagesare by weight unless otherwise indicated. The following test method wasused to characterize the compositions in the following examples.

Test Method

[0039] Solids

[0040] Dried solids of sucrose solutions were determined from measuringthe refractive index of the material and using that measurement tocalculate the solids in weight percent. The refractive index of eachsample was measured with a Leica Mark II Abbe Refractometer (availablefrom Leica Microsystems, Inc., Depen, N.Y.) with the temperature of thewater bath maintained at 25° C. The concentration of each sucrosesolution was calculated from the measured refractive index of thesolution and a correlation between concentration and refractive index.The correlation between concentration and refractive index wasdetermined from data in “Properties of Sucrose Solutions,” CRC Handbookof Chemistry and Physics, 53rd Edition, page D-128, The Chemical RubberCo., Cleveland (1972).

[0041] The following process was used to make the examples.

Process

[0042] The material to be concentrated was placed in a pressurized,jacketed feed vessel kept at 240 kPa (20 psig) and 40° C. This materialwas continuously fed to the evaporation zone using the pressure in thefeed tank and a valve to adjust the flow of liquid. The feed was heatedby passing through a custom-made heat exchanger consisting of a jacketedtube inside an outer tube. The feed passed through the inner tube thathad a length of about 610 mm (24 in) and an outer diameter of about 6.3mm (0.25 in). Temperature controlled water passed through the outer tubethat had a diameter of about 25 mm (1 in). The material next passedthrough a custom-made die containing a single hole or orifice, about1.09 mm (0.043 in) in diameter, before entering the evaporation zone.The evaporation zone consisted of a vertical portion comprising aCLAISEN™ adapter, available as Part No. 5135 from Ace Glass Inc.,Vineland, N.J. A side arm came off the vertical portion and slopedupward to a vertical 10/30 joint, which was connected to a pressuretransducer (Model number 1151AP5E333B1, available from Rosemount, Inc.,Chanhassen, Minn.). Another side arm sloping off the side arm describedimmediately above had a male 24/40 fitting that was connected to a wateraspirator vacuum system (custom-made). The concentrated material wascontinuously removed from the evaporation zone using an air-driven pump(Model FH432 from Viking Pump, Inc., Cedar Falls, Iowa).

[0043] Small jars were filled with the product leaving the evaporationzone. Each jar was filled for 1 minute and then replaced with a nextone. Refractive index of the material in each of the jars was measured.The concentrations of the sucrose solutions were calculated from themeasured refractive index of the solutions and the correlation betweenconcentration and refractive index. The correlation betweenconcentration and refractive index was determined from data presented in“Properties of Sucrose Solutions,” CRC Handbook of Chemistry andPhysics, 53rd Edition, page D-128, The Chemical Rubber Co., Cleveland(1972).

Example 1

[0044] This Example illustrates the response time for a pressure onlychange.

[0045] A sucrose solution (Refractive Index=1.4545, Concentration=64.8wt %) was placed in the feed tank, and flow was begun to the evaporationzone. Conditions were allowed to stabilize at a heat exchangertemperature of 71° C. and an evaporation zone pressure of 9.3 kPa (70torr). The pressure was changed and stabilized at between 5.6 and 5.7kPa (between 42 and 43 torr). After a period of time the pressure waschanged again and stabilized at between 2.8 and 2.9 kPa (21 and 22torr).

[0046] Solids wt % was determined for all samples. Pressure, refractiveindex and calculated solids wt % are shown in Table 1 with elapsed time.TABLE 1 Elapsed Time Pressure Refractive Index Sample min kPa (torr) at25° C. Wt % Solids A 3 9.3 (70) 1.4574 66.1 B 4 Change P 1.4574 66.1 C 55.7 (43) 1.4592 66.8 D 6 5.7 (43) 1.4621 68.1 E 7 5.7 (43) 1.4604 67.4 F8 5.7 (43) 1.4593 66.9 G 9 5.7 (43) 1.4607 67.5 H 10 5.7 (43) 1.460067.2 I 11 5.6 (42) 1.4600 67.2 J 12 5.6 (42) 1.4603 67.3 K 13 Change P1.4605 67.4 L 14 2.9 (22) 1.4603 67.3 M 15 2.9 (22) 1.4646 69.2 N 16 2.9(22) 1.4641 69.0 O 17 2.9 (22) 1.4639 68.9 P 18 2.8 (21) 1.4633 68.6 Q19 2.8 (21) 1.4633 68.6 R 20 2.8 (21) 1.4631 68.5 S 21 2.8 (21) 1.463368.6 T 22 2.8 (21) 1.4632 68.6 U 23 2.8 (21) 1.4631 68.5

Example 2

[0047] This Example illustrates the response time for a temperaturechange only.

[0048] A sucrose solution (Refractive Index=1.4545, Concentration=64.8wt %) was placed in the feed tank, and flow was begun to the evaporationzone. Conditions were allowed to stabilize at a heat exchangertemperature of 57° C. and an evaporation zone pressure of between 5.1and 5.2 kPa (between 38 and 39 torr). The temperature was changed andstabilized at 70° C. After a period of time the temperature was changedagain and stabilized at 83° C.

[0049] Solids wt % was determined for all samples. Temperature,refractive index and calculated solids wt % are shown in Table 2 withelapsed time. TABLE 2 Elapsed Time Temperature Refractive Index Samplemin ° C. at 25° C. Wt % Solids A 11 57 1.4584 66.5 B 12 Change T 1.458466.5 C 13 57 1.4584 66.5 D 14 58 1.4585 66.5 E 15 63 1.4595 67.0 F 16 661.4607 67.5 G 17 68 1.4610 67.6 H 18 69 1.4612 67.7 I 19 69 1.4611 67.7J 20 70 1.4615 67.8 K 21 70 1.4615 67.8 L 22 70 1.4614 67.8 M 23 701.4611 67.7 N 24 70 1.4609 67.6 O 25 70 1.4612 67.7 P 26 Change T 1.461167.7 Q 27 70 1.4618 68.0 R 28 73 1.4616 67.9 S 29 79 1.4623 68.2 T 30 811.4634 68.7 U 31 81 1.4641 69.0 V 32 82 1.4646 69.2 W 33 83 1.4648 69.3X 34 83 1.4645 69.1 Y 35 82 1.4649 69.3 Z 36 84 1.4649 69.3 AA 37 841.4648 69.3 AB 38 82 1.4643 69.0 AC 39 82 1.4643 69.0 AD 40 83 1.464068.9 AE 41 83 1.4644 69.1 AF 42 83 1.4645 69.1 AG 43 83 1.4642 69.0

[0050] The easiest way to compare the dynamics of the changes inExamples 1 and 2 was to plot the normalized change in solidsconcentration versus the time after a deliberate change occured in theset-point of either the pressure or the temperature. FIG. 2 shows thenormalized solids % changes as they relate to changes in the pressure ortemperature for Examples 1 and 2 from the start of each change incondition. As seen, the response to a change in pressure was much fasterthan the response to a change in temperature. One can compensate for theovershoot that occurred with a pressure change using standard methodsknown to those skilled in the art of process control. This would resultin pressure control as a primary means of control of evaporationprocesses that would be much more rapid than that obtainable withtemperature control alone.

What is claimed is:
 1. A process for concentrating weak black liquor toform strong black liquor, said process comprising: a. preconcentratingsaid weak black liquor; b. heating said preconcentrated weak blackliquor in a heating zone under sufficient pressure to prevent saidpreconcentrated weak black liquor from boiling in said heating zone; c.passing said preconcentrated weak black liquor, having at least onevolatile component, into an evaporation zone through one or moreorifices; d. applying pressure in said evaporation zone that is lowerthan the vapor pressure of said preconcentrated weak black liquor as itis passed into said evaporation zone, which allows for flash evaporationof at least a portion of said at least one volatile component of saidpreconcentrated weak black liquor from said preconcentrated weak blackliquor to form strong black liquor; e. adjusting temperature in saidheating zone, pressure in said heating zone and pressure in saidevaporation zone to allow for evaporation of an amount of said at leastone volatile component from said preconcentrated weak black liquor toform said strong black liquor; and f. collecting said strong blackliquor from said evaporation zone.
 2. The method of claim 1 wherein saidevaporation zone has at least one inner surface, and wherein saidpreconcentrated weak black liquor does not substantially contact said atleast one inner surface of said evaporation zone as said preconcentratedweak black liquor passes through said evaporation zone.
 3. The method ofclaim 1 wherein said one or more orifices are shaped such that a desiredsurface area of said preconcentrated weak black liquor is exposed insaid evaporation zone.
 4. The method of claim 1 wherein said pressure insaid heating zone and/or said pressure in said evaporation zone areregulated using one or more pressure sensors.
 5. The method of claim 1wherein the temperature of the preconcentrated weak black liquor priorto said preconcentrated weak black liquor passing into said evaporationzone is regulated using one or more temperature sensors.
 6. The methodof claim 1 wherein a conditions sensor monitors the conditions of thestrong black liquor after it is collected from said evaporation zone. 7.The method of claim 1 wherein conditions or properties of the strongblack liquor are controlled by regulating the pressure in saidevaporation zone and/or by regulating the temperature of thepreconcentrated weak black liquor as it is passed into said evaporationzone.